Process for purifying wood pulp



Patented Dec. 10, 1940.

use STATES PROCESS FOR- PURIFYING WOOD PULP Arthur N. Parrett and Elliott n. woodrue, Shelton, Wasln, assignors to Rayonier Incor Delaware porated, San Francisco, Calif., a corporation of.

No Drawing. Application September 27, 1937, Serial No. 16 5,S28

6 Claims.

This invention relates to a process of manufacture of wood pulp substantially freed of resin and colored matter from woods containing resins of high acid number. More particularly our in- 6 vention or discovery relates to the manufacture oi sulphite wood pulp with a high degree of removal of resins and colored matter from such pulp, so as to render the same highly suitable for the manufacture of rayon, cellulose deriva- 19 tives and line papers.

When sulphite wood pulp is manufactured from highly resinous woods, the unbleached pulp produced contains resinous bodies, waxes, lignin, and

other colored impurities which are only incomw pletely removed by ordinary bleaching methods.

These resins, when present in sulphite pulp, cause...

. which also contains a fairly high proportion of resins, although not as high as the woods mentioned above. All of the woods. mentioned above,

including eastern spruce, contain resins of the acid" type, so called because'these resins have 35 a high acid number. When the term "resin of high acid number is used herein we mean to include wood resins present in sulphite pulp having an acid number above 40.

One method of measuring the extent of the 40 colored materials remaining in pulp produced from these woods is to bleach such pulps by ordinary bleaching methods and then to convert such bleached pulps into nitrocellulose by a standard method. These nitrocellulose products may then be dissolved in well known organic solvents and their color observed. Following this procedure it is found that pulps produced from highly resinous woods tend to yield nitrocellulose 50 solutions of highlycolored character. However,

when such pulps are treated by methods according to ourdiscovery or invention, which largely eliminate the resinous bodies and other colored matters, the nitrocellulose produced from such 55 purified pulps forms an almost water white solution and it is possible to carry such purification to a high degree.

This objection to the presence of colored matter extends to even minute quantities, such as small fractions of a percent. The removal of resins particularly has constituted an obstinate problem. Hence, the discovery of suitable agents to overcome the difllculties set forth has presented amost exacting problem involving fundamental Iactors, respecting which little is chemi- 10 cally known at the present time.

It is old in the art to treat cellulose such as wood pulps, cotton fabrics, etc., with alkaline solutions prior to bleaching in order to remove colored materials, resins, waxes, etc. This art also includes the use of alkaline agents which include soaps to accomplish removal of resins and colored matter. In the commercial production of wood pulp there are objections to the use of agents, such. as soap, because of the danger that small amounts of fatty acids, arising from hydrolysis of the soap, will be precipitated on the fibers and remain in the finished, bleached pulp. Another objection to the use of soaps is the fact that most fatty-acid soaps are partially precipitated as an in soluble calcium soap by the calcium present in the'water, unless the calcium present in the water is extremely low. Finally, soaps are-quite expensive and it is highly desirable touse cheaper chemicals, free from the danger of contamination of the pulp with undesirable materials.

The resinous bodies present in slash pine, loblolly, jack pine, and other woods of the acid resin type, can be recovered by extraction with very effective because the resin is apparently not fully or completely dispersed so as to be readily removed from the pulp on washing. At least numerous attempts to remove resins by caustic soda treatments have proven to be only slightly eflective or hardly effective at all.

The chief object of this invention is to provide a method of purifying pulps particularly from those woods containing resins of high acid numher in such a way that a large portion of the resin and other objectionablev colored materials [will be easily and economically removed and to provide a sequence of steps whereby such a treatment may be economically and efllciently practiced.

Our invention provides that the pulp be treated at high density and mildly elevated temperature with a dilute solution of caustic soda containi a small amount of trisodium phosphate. This treatmenttis preferably carried on in a bleach engine provided with extraordinarily good agitation. The degree of agitation, as well as the strength of the solutions used and the temperature of treatment, are critical factors in the effectiveness of the treatment. We prefer mild temperature conditions in the range 45 C. to 80 C. We find that the strength. of the caustic solution may' be varied to a certain extent. In general, the strength of the solution is a more important matter than the ratio of caustic soda to dry pulp. To effect economy in caustic soda usage and to permit the type of agitation desired, we prefer to use as high a density of pulp as practicable in the'engines where the treatment is given. The degree of agitation necessary may be defined as that required to cause then dispersion promptly occurs. Obviously any type of engine may be used, provided the degree of agitation or circulation is sufilcient to eifect the results specified. One type of engine which may be employed is the well known bleach engine equipped with a vertical worm agitator which is often inclosed in a draft tube. Ordinarily these engines are designed to turn over the mass of pulp once every seven or eight minutes. We prefer to operate these engines so that the rate of turnover is reduced to four or five minutes. Manlfestly, such rate of turnover provides decidedly drastic agitation.

When sulphite pulp is circulated to an extraordinary degree at a high consistency in these engines, the mechanical squeezing and pressing action on the pulp has a marked eflect in intimately distributing the caustic soda and trisodium phosphate through the mass of the pulp ing dispersion is evidenced by this fiufiing. Ob-

viously, the mechanical treatment provided as a desirable feature of our discovery or invention has little or no tendency to impart the characteristics resulting from beating or hydrating pulp. A

We find that stock densities of 3% to 30% of dry stock represent the range of consistencies which can be agitated to give this desired effect but we prefer for reasons already given to work in the range of consistency from 12% to 30% because the mechanical agitation is more drastic and efiective at these high densities. Un-' der these conditions we prefer to maintain the alkalinity of the solution in the range 0.5% to 1.2% caustic soda based on the solution. The amount of trisodium phosphate required to properly disperse the colored materials so that they may be readily washed from thepulp lies in the-range 0.05% to 0.15% anhydrous NaaPOt also based on the solution. Higher proportions of trisodium phosphate may be used but are unnecessary and uneconomical; For a pulp consistency of 25% the caustic soda used, based on the dry weight of pulp, may be varied from 0.7%.

to 5%. On this same basis the trlsodium phosphate expressed as the anhydrous salt may be varied from 0.3% to 1.5% based on the dry weight of pulp. It is obvious that these figures may be changed somewhat as the stock consistency is varied in the range 12% to 30%. In any case, we have discovered that the amount of caustic soda used in the process should be sufficient so that the pH of liquor in the mass of pulp at the end of the treatment should be above 11.0.

We prefer mild temperature conditions in the range already mentioned because in our treatment we do not seek to eflect a purification of the pulp bymeans of dissolving and removing part of the cellulose itself, thereby causing a noticeable increase in the alpha cellulose content of the pulp. A small portion of the caustic soda used in the treatment may be consumed, owing to the fact that sulphite pulp may contain materials of an acid nature which combine with caustic soda. In general, however, the caustic soda used in the process of our invention or discovery remains largely unconsumed and the solution is of high alkalinity at the time the treatment is completed. When the temperature of treatment is too high, the caustic soda attacks the less resistant portions of the cellulose, a chemical reaction which forms the basis of manufacturing alpha pulp by cooking or digesting sulphite pulp with caustic soda. When this reaction of the caustic soda proceeds to. any noticeable extent, we have discovered that the effectiveness of the treatment in dispersing and removing colored materials and resins becomes inhibited, or practically eliminated. In general, therefore, the temperature range in which we prefer to work lies above the temperatures ordinarily used in bleaching and considerably and safely below the temperatures practiced in the manufacture of pulp of increased alpha cellulose content by alkaline digestion or cooking and accordingly, we find that the .process of our invention or discovery operates satisfactorily within the temperature range of 45 C. to 80 C.

The time necessary to effect the desired dispersion and purification will depend somewhat on the temperature of treatment and the degree of agitation. With the agitation and the densities already described, we find that the time may be reduced to a short period, but it is practically desirable to carry on the treatment for water. While some precipitation of tricalcium phosphate might occur when water is used containing 15 to 20 parts per million of calcium, this is not objectionable because we have discovered that the amount of calcium phosphate precipitate absorbed by the pulp will be extremely small and is readily soluble in the acid treatment solutions usually given to the pulp subsequent to the final bleaching step, especially when the pulp is to be used for conversion into rayon, where low ash content is desirable.

The treatment step with caustic soda and tri-.

sodium phosphate, we have discovered, may best be practiced either on the unbleached pulp before any chlorine or hypochlorlte bleaching or immediately after an initial bleaching step with chlorine. In either case we prefer to bleach the pulp with hypochlorite solution after the first two steps, one of which is bleaching. with chlorine and the other the treatment with caustic sodatrisodium phosphate mixture. After final bleaching with hypochlorlte, which may be with sodium or calcium hypochlorite, we prefer to treat the pulp with a solution of sulphur dioxide or some other mineral acid. Such acid treatments are considered to be effective in the removal of inorganic constituents which contribute to the ash content of the pulp. In addition, in this particular case, such acid treatments are very effective in the removal of small traces of calcium phosphate, which may have been absorbed by the pulp due to the presence of calcium in the pulp or in the water used for treating and washing the pulp during the caustic soda-trisodium phosphate treatment. As pointed out immediately above, the ability to readily remove these small traces of phosphate is a distinct advantage of our invention.

The following examples will illustrate the invention:

(1) Slash pine wood was chipped and cooked to. a soft sulphite pulp. The unbleached pulp was washed free of sulphite liquor with water containing about it parts per million of calcium. At this point the pulp contained 1.4% of resinous matter as determined by extraction with ether. It was then thickened to 27% consistency. After thickening, a solution of caustic soda and trido sodium phosphate was added to the pulp. The

addition of chemicals was such that 4% by .weight of caustic soda was 0.75% of anhydrous trlsodium phosphate based on the dry fiber was added. The pulp was then agitated and steamed 45 in a bleach engine of the well known type equipped with a worm agitator and designed to give exceptionally thorough and rapid agitation.

The steam was added gradually over a period of thirty minutes until the temperature reached 50 74 C. The agitation was continued for thirty minutes longer and the mass was then dumped from the engine and washed with water containing about 25 parts per million calcium. After washing the pulp was then bleached, first by' 55 chlorinating and finally by bleaching with calcium phypochlorite solution under normally alkaline conditions. After final hypochlorlte bleaching the pulp was washed, treated with sulphur dioxidesolution and dried. The pulp thus 60 produced was of exceptionally white color and its resinous content as determined by extraction with ether was 0.10%. The finished pulp was converted into nitrocellulose and into.rayon by the viscose process and these products also proved 05 to be of exceptionally white color and high quality.

(2) Loblolly pine was pulped by the sulphite process and was found to contain 1.1% of resinous matter extractable with ether. The first 7 treatment step was direct chlorination, a total of 1.75% of chlorine being added to the pulp. After chlorination the pulp was washed with water containing about 5 parts per million of calcium. The pulp was then thickened to 24% consistency 75 and a solution of caustic soda and trisoduim phosphate was added in the amount of 3% of caustic soda and 1% of anhydrous trisodium phosphate based on the weight of dry pulp. This mixture was then agitated while heating to a temperature of 65 C. The mass was agitated for a total of 90'minutes. At the end of this time the pulp was washed thoroughly with water containing about five (5) parts per million of calcium. At this point the pulp was noticeably whitened by the treatment. The pulp was finally bleached with sodium hypochlorite solution and readily bleached up to a'high white color.

The finished pulp contained 0.09% of ether extractable matter. white.

A portion of the same lot of unbleached pulp used in the above experiment was then bleached by a three stage process consisting of chlorination followed by two stages of bleaching with sodium hypochlorite. This pulp was of consider ably lower colorthan the pulp described aboveand a dilute solution of caustic soda and hisodium phosphate of such concentration that the spent liquor at the end of the treatment shall have a pH of not less than ii; and thoroughly and drastically mechanically agitating the mixture at a temperature of not less than 45 C. until the fibers acquire a flufied and puffed effect.

2. The process of manufacturing from wood containing resins of high acid number a sulphite pulp substantially freed of resin and colored matter comprising forming a mixture of sulphite pulp of 12% to consistency with a solution containing caustic soda and trisodium phosphate, in such proportions that there will be present in the mixture 0.7% to 5.0% of caustic soda and 0.3% to 1.5% of anhydrous trisodium phosphate, each of which percentages is based upon the dry weight of the pulp; and thoroughly and dras= tically mechanically agitating the mixture at a temperature between C. and 80 C. until there is developed in the mass of fibers a fiuffed and pufi'ed efiect.

3.. The process of manufacturing from wood containing resins of high acid number a sulphite pulp substantially freed of resin and colored matter comprising forming a mixture of said pulp of a consistency in the range of 3 to 30% and a dilute solution of caustic soda and triso dium phosphate of such concentration that the spent liquor at the end of the treatment shall have a pH of not less than 11; thoroughly and drastically mechanically agitating the mixture at a temperature of not less than 45 C. until the fibers acquire a fiuffed and pufied efiect: and subsequently bleaching said pulp with al kaline hypochlorlte reagents.

4. The process of manufacturing from wood containing resins of high acid number a sulphite pulp substantially freed of resin and colored matter comprising chlorinating a washed sulphite pulp; forming a mixture of said pulp of a consistency in the range of 3 to 30% and a dilute Nitrocellulose made from this pulp was very "pulp substantially freed of resin and colored matter from wood containing resins of high acid number, the steps in sequence of treating said wood comprising digesting said wood by the sulphite process; washing said sulphite pulp; thickening said washed sulphite pulp to a consistency in the range of 3% to each of which percentages is based upon the dry weight of the pulp; mixing said thickened pulp with a solu- 0 tion of caustic soda and trisodium phosphate of such concentration that the spent liquor at the end of the treatment shall have a pH of not less than 11; thoroughly and drastically mechanically agitating the mixture'at a temperature between- C. and C. until there is imparted to the fibers a flufied and pufied eflect; washing said pulp; bleaching said washed pulp; washing said bleached pulp; and treating said washed pulp with sulphur dioxide solution.

6. In the process of manufacturing a sulphite pulp substantially "freed of resin and colored matter from wood containing resins oi high acid number. the steps in sequence of treating said wood comprising digesting the said wood by the sulphite process; washing said sulphite pulp; chlorinating said washed sulphite pulp; washing said chlorinated pulp; thickening said washed sulphite pulp to a consistency in the range of 3 to 30%, each of which percentages is based upon the dry weight of the pulp; mixing said thick ened pulp with a solution of caustic soda and trisodium phosphate 0! such concentration that the spent liquor at the end of the treatment shall have a. pH of not less than 11; thoroughly and drastically mechanically agitating the mixture at a temperature between 45 C. and 80 C. until there is imparted to the fibers a flufledand puffed efiect; washing said pulp; bleaching said washed pulp; washing said bleached pulp; and treating said washed pulp with sulphur dioxide solution.

ARTHUR N. PARREI'I.

ELLIOTT H. WOODRUFF. 

